Current methods of testing the quality of data signals and data links between a data transmitter and receiver in systems which allow the transmission of video and audio data to allow the generation of television programs available for selection by subscribers at receiving locations generally requires specialist equipment that is not included within the “normal” broadcast data receiver at the subscriber locations. The present invention relates to a method to allow the quality assessment to occur between a transmitting location and a receiving location at which said broadcast data receiver is provided. One broadcast data standard is the Digital Video Broadcasting (DVB) standard and this specifies forward error correction (FEC) methods to attempt to limit the errors which occur in the data transmission and hence improve the video and audio reproduction at the receiver locations. Over a limited range (around the Quasi-Error Free QEF) the bit error rate (BER) can be used as a means of assessing link quality. However this method is only valid around the QEF point.
For example, considering the DVB C standard that uses the Reed Solomon (188,204) FEC method. Typically, below QEF-2.5 dB the BER no longer gives any link quality information (the FEC cannot lock to the signal) and above QEF+2.5 dB a BER measurement takes longer than 1 second for a reliable measurement (e.g. at QEF+5 dB requires over 25 seconds for a reliable BER measurement). Thus to measure Link Quality outside a narrow range around QEF requires a more practical method.
To assess the link quality outside the range covered by the FEC, to measure the BER it is required to measure either Signal to Noise Ratio (SNR) or Modulation Error Ratio (MER). Both these ratios can be calculated from the demodulated signals raw I and Q samples, prior to symbol de-mapping.
The problem in calculating these ratios is to be able to separate out the noise energy from the signal energy. This can be achieved by either measuring the signal for a ‘long time average’ or by transmitting a ‘known signal’ (e.g. a pseudo-random number sequence with known generating function).
The ‘long time average’ method separates the signal from the noise by averaging out the noise component of the signal. This is fairly trivial for QPSK signals but requires an increasing number of samples and is increasingly computationally demanding with higher modulation modes (increasing with number of bits per symbol). If the demodulator is unlocked and the number of bits per symbol is not known, then it is not possible to determine the difference between noise and data at a higher modulation mode.